FIG. 7 is a cross-sectional view illustrating a structure of a conventional hydraulic machine. The hydraulic machine of FIG. 7 is a Francis turbine.
In the hydraulic machine of FIG. 7, a part of a flow path is formed by a casing 1 configured to guide pressured water from the upstream, a stay vane 2 disposed on an inner peripheral side of the casing 1 to rectify the water from the casing 1, and a stay ring 3 configured to interpose the stay vane 2 from above and below. Furthermore, the hydraulic machine of FIG. 7 is configured to guide the water flowing through the flow path to a guide vane 4 which is disposed on the inner peripheral side of the stay ring 3 and serves as a movable vane for flow rate adjustment, and to a runner 5 configured to convert energy of the pressured water into rotational energy.
The runner 5 has a plurality of blades 5a arranged in a ring shape, a crown 5b connected to the blades 5a from the upper side, having a ring shape, and connected to a main shaft 9, a band 5c connected to the blades 5a from the lower side and having a ring shape, and a runner cone 5d provided at a lower end of the crown 5b. The runner 5 is housed between an upper cover 6 and a lower cover 7. The water used to drive the runner 5 is discharged to a draft pipe 8 located downstream of the runner 5, and is discharged to a drainage path via the draft pipe 8. Furthermore, the main shaft 9 is connected to a rotor shaft of a generator 10, and supplies the generator 10 with a driving force for power generation by transmitting the rotational energy of the runner 5 to the generator 10.
Generally, the blades 5a of the runner 5 are fixed. When the output of the hydraulic machine is changed, the flow rate is adjusted by varying the degree of opening of the movable guide vane 4. For that reason, even if an inflow angle of the water flow to the runner 5 changes due to a reduction in the water level or the like of a dam, a situation in which all of the energy of the water flow in the runner 5 cannot be converted occurs since it is not possible to make the blades 5a of the runner 5 movable. As a result, swirl flow flows out of the outlet side of the runner 5. In particular, this phenomenon appears prominently at the time of partial load operation of a small flow rate, and a large spiral vortex 11 due to the swirling flow is generated within the draft pipe 8 in the vicinity of the outlet of the runner 5. Pressure drops significantly in the central portion of the vortex 11, and a cavity filled with water vapor and free air is generated. The bubbled vortex 11 whirls within the draft pipe 8, and therefore, the water pressure pulsation occurs.
The relation between the water pressure pulsation and the flow rate is illustrated in FIG. 8. FIG. 8 is a graph illustrating water pressure pulsation characteristics of the conventional hydraulic machine. From FIG. 8, it can be seen that two regions 12 and 13 having the increased water pressure pulsation exist in a region less than the rated flow rate. Accordingly, the flow rate becomes smaller than the rated flow rate due to a decrease in the water level of the dam or the like, and a large water pressure pulsation occurs when becoming a flow rate in the regions 12 and 13.
It has been reported that the magnitude of the water pressure pulsation of the region 12 is dependent on the strength of the vortex 11, and becomes maximum around a flow rate of about half of the rated flow rate due to the nature of the swirling flow that is a factor of the vortex 11.
In contrast, in the previous visualization studies, it has been known that, as illustrated in FIGS. 9A and 9B, the water pressure pulsation of the region 13 is a synthesis of a rotation mode in which a cross-sectional shape of the vortex 11 is an ellipse 14 and rotates with respect to the spiral axis, and an expansion/contraction mode in which the whole generation regions of the vortex 11 expand and contract in the vertical direction. FIGS. 9A and 9B are a top view and a cross-sectional view for describing the rotation mode and the expansion/contraction mode.
Furthermore, regarding the spiral vortex 11 having the elliptical cross-section that causes the water pressure pulsation of the region 13, it is estimated that the spiral vortex 11 having the elliptical cross-section is formed in the form illustrated in FIGS. 10A and 10B based on a phenomenon analysis using the recent flow analysis. FIGS. 10A and 10B are a cross-sectional view and a side view for describing a generation mechanism of the spiral vortex 11 having the elliptical cross-section. FIG. 10B is a side view of a region 15 of FIG. 10A, and illustrates a cylindrical wall surface of the runner cone 5d. 
As illustrated in FIG. 10B, in the flow field in the vicinity of the wall surface of the runner cone 5d, a centrifugal force 16 caused by the inclination of the wall surface of the runner cone 5d and a dynamic pressure 17 in a main flow direction are major fluid forces. Both of the fluid forces are balanced in the above-mentioned region 13, and a local recirculation region 18 is formed. Since the recirculation region 18 is formed in the vicinity of the wall surface the runner cone 5d in a state of being pressed by the main flow, the recirculation region 18 is formed in an elliptical shape. The recirculation region 18 flows down to a lower zone of the runner 5, and therefore, the spiral vortex 11 having the elliptical cross-section is formed.
Furthermore, regarding the expansion/contraction mode in which the whole generation regions of the vortex 11 expand and contract in the vertical direction, as illustrated in FIG. 11, it is believed that the expansion/contraction mode is caused by the balance between a force 19 by which the vortex 11 attempts to expand to the downstream side by the dynamic pressure of the outlet flow of the runner 5, and a force 20 attempting to return the vortex 11 to the upstream side by the pressure recovery effect of the draft pipe 8. FIG. 11 is a cross-sectional view for describing a generation mechanism of the expansion/contraction mode.